JPS6257512B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit for energizing the print head of an electrical thermal printer.

One type of thermal printer uses a ribbon that internally generates localized heat in response to electrical signals. At this time, the localized heat serves to form a mark on the receiving medium. Typically, electrical signals are applied by print head electrodes that thread across the outer layer of the ribbon, and this outer layer is characterized by moderate resistivity. These signals travel inward to the highly conductive layer (preferably a layer of aluminum), causing localized heating in the process. The signal path is completed by a contact that engages the conductive layer (see, for example, US Pat.
2,713,822) or alternatively by a moderately conductive layer on a current collector plate (see, eg, US Pat. No. 3,744,611) and in which electrical contacts are established.

With this type of printer, a signal applied to the electrodes of the printing head causes the ribbon to heat up, which in turn causes a mark to be formed. The marks can be generated due to the thermal responsiveness of the paper itself, or by transferring a portion of the thermally transferable ink layer on the outside of the ribbon, as is known in the art.

For such resistive ribbon printers, print quality has exhibited undesirable fluctuations when the electrodes are driven by selectively applying a constant voltage.

However, it has been found that mark formation of satisfactory quality can be achieved by using selectively triggerable current sources to drive each electrode of the printing head (IBM Technical Disclosure
Bulletin, Vol. 22, No. 2, pp. 790 and 791).

A disadvantage of the constant current method for driving the electrodes of a printing head arises because a gated individual drive circuit is provided for each electrode, thereby increasing the overall circuit complexity and energy consumption.

Because the current drive is regulating the current rather than simply switching, a significant amount of energy is dissipated, which makes it possible to reduce the cooling requirements for low-cost, miniaturized equipment, such as integrated circuit chip configurations. cause difficulties.

The present invention includes the recognition that an important influence on print quality variations for resistive ribbon printers is the voltage drop in the signal return path that includes a highly conductive layer "embedded" in the ribbon. Furthermore, if an electrical contact engaging the ribbon at the surface of the resistive outer layer is used in connection with a high impedance monitoring circuit, a voltage substantially corresponding to the voltage at the embedded conductive layer will be present at this contact. You can see that it can be monitored.

By monitoring the ribbon voltage in this high impedance circuit, only a small monitoring current flows so that the potential set by the printing current is
The monitoring current is not appreciably changed by the resulting ohmic voltage drop. If the monitoring point is moved away from the printing head, the movement of the printing current towards the highly conductive layer does not significantly affect the monitoring voltage and substantially corresponds to the voltage of this conductive layer at the printing point. It becomes possible to generate a feedback voltage that Most of the drive signal current flows in one direction along the ribbon away from the print head and the monitoring contact area ensures that any potential drop resulting from the flow of printing current in the highly conductive layer is brought to this monitored potential. Preferably, the ribbon is disposed on the ribbon at a distance from the printing head in diametrically opposed directions so as to include the printing head. By using this feedback signal, the drive voltage applied to the electrodes is modified to reduce the sensitivity of the printing process to voltage drops in the current return path. Preferably, the feedback signal is used to modify the applied drive voltage to effectively offset the voltage drop in the current return path.

Preferably, the feedback circuit acts on the applied voltage before the switching gate that selects the respective electrode, so that only one drive signal source is required. Equal sized resistors may be placed in series with the individual electrodes to promote uniformity of current flow.

In a preferred embodiment of the invention, the drive signal return contact comprises a conductive roller located on the ribbon take-up side of the print head and the electrical monitoring contact comprises a conductive roller located on the ribbon take-up side of the print head. located on the ribbon supply side of the

The environment of the present invention is first considered in the context of prior art constant current drive circuits for electrode energization.

Referring to FIG. 1, a print head 10 rubs or scans along a resistive ribbon 12 which contacts a receiving medium, such as paper, and marks are formed on the receiving medium. 1 set of printed electrodes 16 (hereinafter "N" electrode sets are assumed)
contacts the resistive ribbon 12 in the printed area and such contact occurs at the surface of the layer 18 of moderate resistance (e.g., a resistance profile in the range of 200 to 400 ohms per 6.45 ^cm2 is preferred, but a wider range of (gives the possibility of satisfactory performance even at low values). Adjacent to resistive layer 18 is a thin conductive layer 20, which is preferably a thin layer of aluminum. An outer ink layer 22 of thermally transferable ink is typically formed adjacent conductive layer 20 . However, if the receiving medium 14 is heat sensitive, the outer ink layer 22 is not necessary to form the mark.

In operation, the print head energizing means 24 applies signals (designated D ₁ through D _N ) to the print head 10 via a set of electrode wires or channels 25 to cause marks to be formed on the receiving medium 14 . Apply. A known technique for achieving an acceptably uniform print quality is to use individual constant current drives 26 for each electrode 16 (denote this current by I _K and to indicate a good direction of normal current flow). (indicated by an arrow). The current driver 26 is energized by a power supply signal designated Vs and has a gate signal (G ₁
(denoted as G) and this gate signal causes the signal D to be selectively applied to the electrodes 16.

The signal D applied to the electrode 16 causes the resistance ribbon 1
2 tends to pass through the moderately resistive layer 18 to the conductive layer 20 and cause localized heating in the process. Mark formation results from localized heating, either by transfer of a portion of the ink layer 22 or by alteration of the receiving medium 14 (eg, thermal paper). The signal path of the signal D extends primarily through the conductive layer 20 to the current collection area where the current collection contact means 28 engage the ribbon 12. As shown, the current collection contact may be a conductive roller that engages the moderately resistive layer 18 and cooperates with the pressure roller 30 to achieve intimate electrical contact. The current collecting contact means 28 is electrically connected via a low impedance connection 31 to provide a signal return path to the biasing means 24.
The low impedance connection 31 may be a ground connection or a directly wired connection that includes a portion of the printer frame (not shown).

Gating signal G, which controls the time period for selectively generating signal D, is generated by printer controller 32, which cooperates with font storage 34 as is known in matrix printers. work It will be appreciated that this configuration requires a separate current driver 26 to provide a current regulating operation that involves significant heat generation.

Referring to FIG. 2, print head energizing means according to a preferred embodiment of the present invention includes monitoring contact means 50.
The monitoring _contact means is preferably an electrically conductive roller which cooperates with the pressure roller 52. The monitoring contact means or roller is preferably located on the path of the ribbon 12 at a location opposite the print head 10 from the drive signal current collection contact means 28. By arranging the monitoring contacts in this manner, it is possible to monitor the voltage level, which is essentially the voltage across the conductive layer 20 in the printing area (printing head 10), as will be explained below.

To facilitate explanation of this ability to monitor embedded layers, a simplified and summarized variable representation of ribbon 12 will be described with reference to FIG. A set of resistors 100 represents the path resistance between electrode 16 and highly conductive layer 20. The resistance of the highly conductive layer 20 between the printed area and the contact area of the monitoring contact means 50 is represented by resistor 102 and the resistor 104 provides the resistance to the monitoring contact means 50 through the layer 18 of medium resistance. express.

In the opposite direction, the resistance of the highly conductive layer portion extending from the printing area to the contact area of the contact means 28 is represented by a resistor 106. resistor 108
represents the resistance through the layer 18 of moderate resistance in the contact area of the current collecting contact means 28. Other signal paths can also be represented according to the ribbon resistance distribution, but these paths are less important than the paths described above with respect to the voltage level in question.

Due to the fairly high impedance of the monitor contact means 50, the current of the drive signal D is reduced to the current collecting contact means 2.
It can be seen from this figure that this current collecting contact means provides a low impedance connection primarily following the path through resistors 106 and 108 to 8 and back to the energizing means 24'. This current flow of drive signal D establishes a voltage at node 110, which corresponds substantially to conductive layer 20 in the printing area. Due to the high impedance connection to the monitoring contact means 50, a small current flows through the resistors 102 and 104.
flows through it and causes a voltage drop,
Voltage signal V _FBK substantially corresponds to the voltage at node 110.

Although the above explanation is not exact, it is believed to aid in understanding the mechanism and by which a meaningful signal S _FBK is obtained. Moreover, the connection means 31
It will be appreciated that a monitoring contact means 50 is placed on the ribbon path to permit monitoring of the total voltage drop from the conductive layer 20 in the printing area through and back to the biasing means 24'. That is, this is best achieved by locating the monitoring contact means 50 on the opposite side of the printing head 10 from the current collecting contact means 28. Preferably, as shown, the monitoring contact means 50 is on the supply side of the ribbon 20 and the current collection contact means 28 is on the take-up side of the ribbon. The monitoring contact means 50 is spaced from the printing head 10 so that there is little or no potential influence on the monitored potential resulting from the movement of the printing current through the moderately conductive layer 18.

Referring to FIG. 2, the signal _{S_FBK} from the monitoring contact means 50 is applied to the monitoring means 200. This monitoring means is connected to a pair of resistors 204 and 20 to act as a high impedance analog buffer.
Preferably, the operational amplifier 202 is connected to 6 (the good resistance value of this example is shown).

Reference voltage V _REF is applied to analog buffer 208, which is preferably an operational amplifier 210 connected to a pair of resistors 212 and 214 to act as a high impedance analog buffer. The reference signal V _REF can be applied by an operator adjustable potentiometer 215, but may alternatively be applied by a controller such as a programmed microcomputer (not shown). The signals from monitoring means 200 and buffer 208 are processed by means such as summing circuit 216. The summing circuit is preferably an operational amplifier 218 with two input resistors 220 and 222 and a feedback resistor 224 connected to the input summing junction. The voltage from buffer 200 serves as a buffered feedback to nullify all or a portion of the ribbon voltage transmitted to monitoring contact means 50 in accordance with the present invention. The balance of the responses to signals S _FBK and V _REF is controlled by the relative sizes of resistors 220 and 222 (for this preferred embodiment, equal resistances are used) and the multiplier effect of the sum is the feedback resistor. controlled by the size of container 224.

Amplifier 218 provides energizing signal S _E to selection circuit 226.
The selection circuit includes a balancing resistor 228 and a single control switch transistor 230 for each separate electrode channel 25. The balancing resistor acts to average the current flow between these channels 25 and these switches 2
30 selectively passes drive signal D in accordance with timing signal G generated by print controller 32 as described above.

When the feedback method described above is used to adjust the electrode bias in a resistive ribbon thermal printer, it provides satisfactory print quality without the need to individually adjust the bias for each electrode as is the case with constant current drive. It is permitted to give

[Brief explanation of the drawing]

FIG. 1 is a partial block diagram showing an electrode driving configuration in a resistive ribbon printer, FIG. 2 is a partial circuit diagram showing a favorable electrode biasing configuration of the present invention in a resistive ribbon printer, and FIG. FIG. 6 is a diagram useful in explaining current flow in a preferred electrode energization configuration of the present invention. 12... Ribbon, 18... Resistance layer, 20... Conductive layer, 22... Ink layer, 16... Printed electrode, 2
5... Electrode channel, 32... Printer control device, 34... Font storage device, 28... Current collecting contact means, 31... Connection body, 50... Monitoring contact means, 200... Monitoring means, 208... analog·
Buffer, 218...amplifier, 215...potentiometer, 226...selection circuit.

Claims (1)

Claims: 1. A thermal printing ribbon having an outer layer of moderate conductivity and an inner layer of high conductivity is used, and includes a printing head with a set of electrodes, which electrodes are arranged in the printing area to applying a respective printing signal to rub on the conductive outer layer and locally heat the ribbon and generate a mark on the receiving medium, the printing signal being in accordance with a gating signal from a printing controller; In a print drive circuit for use in printers of the timed type, current collection contact means for establishing electrical contact with said moderately conductive layer of said ribbon in a current collection area spaced from said printing area; monitoring contact means for establishing electrical contact with said moderately conductive layer of said ribbon in a sensing area spaced from said printing area and said current collecting area; and a buffer representing the potential provided by said monitoring contact means. a high impedance circuit electrically connected to said monitoring contact means for generating a feedback signal;
a signal channel connected to each of said electrodes, signal processing means responsive to said buffered feedback signal to generate a set of print signals, said buffered feedback signal to generate an energizing signal; comprising means for adjusting a reference voltage level in accordance with a signal, and selectively connecting said energizing signal to said signal channel in accordance with said gating signal to serve as a respective printing signal at said electrode. means for defining a low impedance current return path between said current collecting contact means and said print head energizing means so as to establish a return path for said print signal; A thermal printer drive circuit consisting of: